In the manufacture of wood pulp suitable for use in the paper industry or in the manufacture of other wood fiber containing products one of the first steps involves removing the bark from the logs. This can be done using a dry method in which the bark is removed with a debarking drum and the removed bark fines are collected with conventional dust collection type equipment. Because of economical and environmental problems with the dry debarking process, it is preferred to use a wet debarking process in which the bark fines are removed as an aqueous suspension. All or part of the aqueous suspension is recirculated in the wet debarking process until such time that the solids or bark fines concentration in the suspension become undesirably high. At this point the suspension must be dumped or treated in some manner to reduce the solid content below a tolerable level. The bark fines in the aqueous suspension range in size from sub-micron to approximately 1/4 inch long, and can also contain ropy strings of agglomerated particles and/or fibers which are even much longer. This suspension also contains substantial amounts of pitch and resin particles removed from the outer layers of the logs.
One attempt to remove the bark fines from the aqueous suspension in the wet process was to pass the suspension over a conventional water screen to screen out the solids or bark fines from the liquid carrier. This attempt was unsuccessful because the shape of the bark fines particles along with the pitch and resin contained in bark fines resulted in rapid blinding over of the screen, i.e., clogging of the openings in the screens, in such a manner and so seriously that this blinding problem could not be overcome.
Another method presently used to remove the bark fines from the recirculated suspension is to continuously bleed off a portion of the recirculating suspension and replace the amount bled off with fresh water. That portion of the suspension bled off of the system is put into large settling ponds where the higher density solids settle out of the suspension. The liquid carrier, still containing a substantial amount of bark fines, is then fed to a waste water biological treatment plant or activated sludge system for removal of the bark fines and reduction of the biodegradable content to below a suitable level for disposal. This method of removing the bark fines from aqueous suspensions frequently constitutes the major influent loading to waste water biological treatment plants in pulping operations, and is a costly manner for removing the bark fines. In fact, its cost is so significant that it often exceeds the other advantages of the wet process over the dry process of debarking.
An economical manner of solving this problem is to divert at least a portion of the bark fines suspension to a rotary vacuum precoat filter which will remove the bark fines from the suspension producing a filtrate suitable for blending back in with the remainder of the bark fines suspension for recirculation as a purge flow to the debarking operation. This method is described in U.S. Pat. application Ser. No. 576,078 filed on May 9, 1975 in the name of George Richard Bell, the disclosure of which is herein incorporated by reference. While this technique works fine, after a prolonged period of operation the resin and pitch in the bark fines and water foul the equipment and cause partial blinding of the cloth septum whenever one or more cracks develop in the precoat layer.